Extract data elements found in a single column

Here is what my data look like.

id interest_string
1       YI{Z0{ZI{
2             ZO{
3            <NA>
4             ZT{

As you can see, can be multiple codes concatenated into a single column, seperated by {. It is also possible for a row to have no interest_string values at all.

How can I manipulate this data frame to extract the values into a format like this:

id  interest
1    YI
1    Z0
1    ZI
2    Z0
3    <NA>
4    ZT

I need to complete this task with R.

Thanks in advance.